1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly, to semiconductor photo detecting devices for an application of optical fiber communication systems and optical information processing devices, and manufacturing method of the device.
2. Description of the Related Art
Semiconductor photo detecting devices convert a photo signal into an electric signal, and are essential components in optical fiber communication systems. A recent increase in traffic through optical fiber communication systems, accompanied by an increase in communication speed, demands a further increase in a response speed of semiconductor photo detecting devices.
PIN photo diodes are widely used as conventional high speed semiconductor photo detecting devices. A PIN photo diode includes a photo absorption layer where an incoming photo signal generates photo-excited carriers thereof. The carriers are output as a photo-electric current through a reverse biased p-n junction. The PIN photo diode is capable of high speed response.
A response speed of the PIN photo diode is, however, restricted by a parasitic capacitance of the p-n junction. Various configurations claiming to reduce the parasitic capacitance are proposed successfully.
As configuration improvements reduce the parasitic capacitance of the PIN photo diode, a carrier movement time in which photo-excited carriers move through the photo absorption layer is drawing attention as another restriction in response speed. By reducing a thickness of the photo absorption layer of the PIN photo diode, the carrier movement time can be reduced, but more incoming photo signal passes through the layer. Since the incoming light is not absorbed sufficiently, a less efficiency of photo detection becomes another problem.
To avoid this dilemma, a PIN photo diode which receives a photo signal at an inclined incidental angle to the slim photo absorption layer to reduce the carrier movement time and, at the same time, to increase the photo detection efficiency, is proposed.
FIG. 1 shows a configuration of a high speed PIN photo diode 10 using a conventional technology.
As shown in FIG. 1, the PIN photo diode 10 is formed on a semi-insulating InP substrate 11, and includes an n-type InP buffer layer 12 grown by epitaxial growth technique on the InP substrate, a non-doped or nxe2x88x92-type InGaAs photo absorption layer 13 grown by epitaxial growth technique on the buffer layer 12, a p-type InP cap layer 14 grown by epitaxial growth technique on the photo absorption layer 13. An n-type ohmic electrode 12A is formed on the n-type InP buffer layer 12, and p-type ohmic electrode 14A is formed on the p-type InP cap layer.
In case of the configuration shown in FIG. 1, the n-type InP buffer layer 12 forms a pattern of a limited area on the InP substrate 11, and the photo absorption layer 13 and p-type InP cap layer 14 also form a mesa structure of a limited area on the n-type InP buffer layer 12, and a parasitic capacitance of the configuration is consequently minimal. Further, in case of the PIN photo diode, an incoming photo signal 1 comes to a bottom face 11A of the InP substrate 11 at an inclined incidental angle of xcex8o, and is refracted at a refraction angle xcex8i. The photo signal passes through the photo absorption layer 13 at an inclined angle.
In case of a photo diode that requires an incoming photo signal passing through the substrate bottom face 11A at an inclined angle, even if the incoming photo signal comes to the substrate bottom face 11A at a big incidental angle xcex8o, the photo signal passes through the photo absorption layer 13 substantially perpendicularly due to a very high refraction rate, about 3.0, of the InP substrate 11. An optical path in the absorption layer 13 is not long enough.
FIG. 2 shows another configuration of a high speed PIN photo diode 20 using a conventional technique, which is described in a Japanese Patent Laid-open Application No. 11-135823.
As shown in FIG. 2, the PIN photo diode 20 is formed on a semi-insulating InP substrate 21, and includes an n-type InP buffer layer 22 grown by epitaxial growth technique on the InP substrate 21, an nxe2x88x92-type InGaAs photo absorption layer 23 grown by epitaxial growth technique on the InP buffer layer 22, a n-type InP cap layer 24 grown by epitaxial growth technique on the photo absorption layer 23. A doped p-type diffusion region 25 is formed in the InP cap layer 24 and a portion of the InGaAs photo absorption layer 23 thereof.
A p-type ohmic electrode 26 connected to the p-type diffusion region 25 is formed on the InP cap layer 24, and n-type ohmic electrode 27 is formed on an n-type region outside the p-type diffusion region 25. An exposed surface of the InP cap layer 24 is covered by a passivation film 24A such as SiN.
In case of the PIN photo diode 20 shown in FIG. 2, a portion of semiconductor layers 22-24 including the substrate 21 is removed by etching from a side. The PIN photo diode 20 has a side face 21A of the substrate 21 and a slope 21B connected to the side face 21A and cutting the semiconductor layers 22-24 at an inclined angle. If a photo signal 1 comes to the slope 21B in parallel to the substrate 21, the photo signal 1 is refracted at the slope 21B toward the photo absorption layer 23.
A configuration similar to the PIN photo diode 20 shown in FIG. 2 is described in a Japanese Patent Laid-open Application No. 11-307806.
FIG. 3 shows a configuration of a PIN photo diode 30 described in the Japanese Patent Laid-open Application No. 11-307806. In FIG. 3, portions described previously are referred by the same numerals as before.
As shown in FIG. 3, the PIN photo diode 30 is formed on a n-type InP substrate 31, and includes an nxe2x88x92-type InGaAs photo absorption layer 32 grown by epitaxial growth technique on the substrate 31, and an n-type InP cap layer 33 grown by epitaxial growth technique on the photo absorption layer 32. A p-type doped diffusion region 34 is formed in the InP cap layer 33 and a portion of the InGaAs photo absorption layer thereof. A p-type ohmic electrode 35 is formed on the p-type diffusion region 34, and n-type ohmic electrode 36 is formed on a bottom principal plane 31A of the InP substrate 31.
In case of the PIN photo diode 30 shown in FIG. 3, a slope 31B is formed at the bottom of the InP substrate 31. A photo signal 1 coming in parallel to the bottom face of the substrate 31 is refracted toward the photo absorption layer 32.
It should be noted that a direction of an incoming photo signal can be changed not only by refraction but also by a reflection.
FIG. 4 shows a configuration of a PIN photo diode 40 described in a Japanese Patent Laid-open Application No. 2000-183390. This configuration utilizes a reflection of an incoming photo signal.
As shown in FIG. 4, the PIN photo diode 40 is formed on a semi-insulating InP substrate 41, and includes an n-type InP buffer layer 42 grown by epitaxial growth technique on the InP substrate 41, an nxe2x88x92-type InGaAs photo absorption layer 43 grown by epitaxial growth technique on the buffer layer 42, a n-type InP cap layer 44 grown by epitaxial growth technique on the photo absorption layer 43. A p-type diffusion region 45 is formed in the InP cap layer 44 and a portion of the InGaAs photo absorption layer 43 thereof. A p-type ohmic electrode 46 is formed on the p-type diffusion region 45, and n-type ohmic electrode 47 is formed on the n-type region of the InP cap layer 44.
A concavity 41A shaped by a slope is formed on the bottom principal plane of the InP substrate 41. An incoming photo signal 1 passes through a side face of the InP substrate 41 in parallel to the bottom principal plane, and is reflected toward the photo absorption layer 43 by the slope shaping the concavity 41A. The concavity 41A is covered by a SiN film 41a and an Al reflection film 41b to increase a reflection rate of the concavity 41A. The Al film 41b is protected by a Ti adhesive film 41c and an Au film 41d. 
As described above, all PIN photo diodes shown in FIGS. 2-4 increase an optical path length in a photo absorption layer, by making an incoming photo signal pass through the photo absorption layer at an inclined angle by refraction or reflection of the incoming photo signal. In these conventional configurations, however, effective efficiency depends on a polarization of the incoming photo signal due to polarization dependency of the refraction or the reflection. Polarization of photo signals propagating through an optical fiber rotates at random. The photo detecting efficiency changes every moment, and the change causes a problem in an optical fiber communication system.
The slope which refracts or reflects the incoming photo signal in the conventional PIN photo diodes described above is formed as a portion of a semiconductor layer occupying a limited region. An incidence of the incoming photo signal must be controlled precisely. Especially, in the case that the p-type ohmic electrode is formed on the photo absorption layer, the positional relationship between the photo absorption layer and the slope must be controlled precisely, and the control makes a manufacturing of the photo diode complicated.
Furthermore, in case of the conventional PIN photo diodes previously described, the slope which is inclined to the principal plane of the substrate is formed by a selective etching process or a dicing process. It is difficult to form an optically flat plane by the selective etching and the dicing. A diffusion loss may be caused by the refraction or the reflection.
Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor photo detecting device in which one or more of the problems described above are eliminated.
Another and more specific object of the present invention is to provide a high speed semiconductor photo detecting device with a low optical loss which does not require a complicated manufacturing processes, and to provide a method for making the same.
In order to achieve the above objects according to the present invention, a semiconductor photo detecting device, includes a semiconductor substrate having a flat side face, and a photo absorption layer formed on said semiconductor substrate, wherein an entire part of said flat side face is inclined to a line perpendicular to a principal plane of said semiconductor substrate, and said flat side face is substantially perpendicular to an incoming photo signal.
At least one side face of the semiconductor substrate entirely is inclined to the principal plane of the semiconductor substrate. By using the entire side face as an incidental face of an incoming photo signal, the incoming photo signal arrives substantially straight at the photo absorption layer formed on the substrate without refraction or reflection. Because the entire side face of the semiconductor substrate is used as the incidental face of the incoming photo signal, the incoming photo signal goes straight to the photo absorption region, and therefore, a position of the photo detection region need not be controlled precisely. Manufacturing process of the semiconductor photo detecting device is simplified. By using an inclined substrate as the semiconductor substrate, the flat side face is easily achieved by simple cleavage operation. It is easy to obtain a side face with sufficient optical quality. Since the incoming photo signal passes through the incidental face perpendicularly, the problem of polarization dependency caused by a refraction or reflection is also eliminated.
Other objects, features, and advantages of the present invention will be more apparent from the following detailed description when read in conjunction with the accompanying drawings.